Our body's largest organ, the skin, is a vital barrier against environmental pressures such as thermal changes, sunlight, allergens, toxins, and microbes. This complex organ is composed of an array of many cell types including epithelia, fibroblasts, neurons, and vasculature that act in unison to ensure its function. Cells of the immune system have emerged as orchestrators of many facets of skin biology that collectively serve to reinforce the barrier against external threats. The composition, function, and activation status of immune cells is constantly tuned by skin structures, microbial and inflammatory stimuli. In turn, skin-resident and recruited immune cells engage in an active dialogue with the tissue parenchyma to regulate commensal bacteria, limit microbial invasion and direct inflammation and repair. Previous research program primarily focused on the functions and biology of Langerhans cells (LC), antigen presenting cells unique in the epidermis, who's in vivo functions had remained unclear since their discovery. We demonstrated that LCs were equipped with a unique mechanism to gain access to foreign antigens that had breached the skin's outermost barrier, the stratum corneum, but that were still outside of epidermal tight junctions (Kubo et al, JEM 2009). This led us to hypothesize that LCs survey for commensal bacteria which may also be pathogenic under certain circumstances. To address this, we inoculated Staphylococcus aureus-derived toxin onto intact skin. While this caused no skin inflammation, we found that LCs acquired the toxin through intact tight junction barriers and induced the production of neutralizing antibodies that protected mice from systemic challenge of the toxin in experimental Staphylococcal scalded skin syndrome (Ouchi et al, JEM 2011). We further demonstrated that LCs took up an epidermal autoantigen to induce the expansion of regulatory T cells that suppressed autoimmune skin disease (Kitashima et al, eBiomedicine, 2018). These studies, in aggregate, established crucial roles of LCs during host defense and autoimmunity. An important accomplishment that led to the current theme of tissue-immune crosstalk was elucidating how the LC network is maintained by hair follicles, one of the defining features of mammals. We discovered that hair follicles are immunologically active structures that, upon sensing mechanical stress, produced chemokines to attract LC precursors and served as a gateway for their repopulation into epidermis (Nagao et al, Nat Immunol 2012). This finding represented a novel concept of how tissue-specific signals communicate with immune cells to maintain immunological homeostasis. We have recently expanded on the above findings by demonstrating that hair follicles produced cytokines that enabled the persistence of memory T cells in the epidermis. Importantly when resident memory T cells underwent malignant transformation to lymphoma, they remained dependent on hair follicle-derived cytokines (Adachi et al, Nat Med 2015). We also studied host-microbe interactions in a disease setting by generating an ADAM17 cKO mouse model of atopic dermatitis (AD) that spontaneously developed eczematous skin inflammation associated with dysbiosis (imbalance of the bacterial flora) that was predominated by S. aureus, a feature that recapitulates human AD. Whether S. aureus colonization on AD skin contributed to eczematous inflammation or was merely a result of chronic inflammation had been debated. We determined in our AD mouse model that S. aureus was a crucial component of eczema formation, providing an answer to a long-standing clinical question (Kobayashi et al, Immunity 2015). Using bulk RNA-seq analysis of skin from ADAM17 cKO mice we demonstrated that the transcriptome in these mice recapitulates that of human AD (Woodring et al, J Invest Dermatol, 2018). We further demonstrated that innate lymphoid cells (ILC) in the epidermis rely on hair follicle-derived cytokines and chemokines for persistence and localization near the sebaceous glands. There, the ILCs regulate the functions of sebaceous glands to tune the equilibrium of microbes that reside on skin surface (Kobayashi et al, Cell, 2019). In the Cutaneous Leukocyte Biology Section at NIAMS, we further explore fundamental mechanisms that underlie tissue-immune and host-microbe interactions during homeostasis and inflammation. These processes involve innate and adaptive immune cells, and we hypothesize that altered crosstalk between epithelial or stromal compartments is involved in the pathophysiology of skin diseases. We employ clinically relevant models, including mouse models for AD, to expand our fundamental knowledge of tissue-immune crosstalk and to build a foundation that will promote better understanding of skin immunity and inflammatory skin diseases.